DIESTIANo 
57 
and the Coralline Crag, if not absolutely contemporaneous, are 
so closely allied that it is impossible to refer them to different 
portions of the Pliocene period. Turning next to the 22 species 
still to be accounted for, 9 of the forms that are unknown in 
the Coralline Crag are still living; and 5 are extinct molluscs 
known to occur in Upper Pliocene beds. Against these we 
have to place 2 south European Pliocene and Miocene shells. 
Area diluvii and Terehra acuminata ; 3 generally considered to 
be characteristic Miocene fossils. Conus Dujardini (a common 
Belgian Miocene fossil, also found in the ‘‘ box-stones ’’ at the 
base of the Crag), Pleurotoma consobrina and Pleurotoma Jouan- 
neti (both characteristic Italian Miocene fossils); and 3 species 
apparently new which are confined to Lenham. Thus the 
Diestian fauna includes 14 species which point to its being newer 
than the Coralline Crag, and 8 species which point to its being 
older. If, however, our acquaintance with the whole Miocene 
fauna were as extensive as with the recent mollusca, this small 
difference would probably disappear, for it is more probable that 
several additional Diestian forms will be discovered in Miocene 
beds, than that a corresponding number of supposed extinct shells 
will be found still to be living. The foregoing list, instead of 
showing that the Diestian sands belong to a Mio-Pliocene forma¬ 
tion older than the Coralline Crag, as was formerly thought, points 
to its being slightly newer, but the difference is so extremely 
small that it may safely be ignored. 
Leaving statistics—never very satisfactory when the relative 
age of closely allied deposits is to be inquired into—there are 
some slight changes in the distribution of the species as one 
travels from north-east to south-west which strongly suggest 
warmer seas in the latter direction. Probably even in Pliocene 
times the Gulf Stream warmed the western shores, and four de¬ 
grees further west would then as now no doubt involve a distinct 
rise in the temperature of the water. 
The Diestian beds of Holland have only been reached in 
borings, so that a comparatively small number of fossils has yet 
been obtained from them. These, however, have been carefully 
collected and described by Dr. Lorid, who kindly showed me the 
original specimens during my visit to Utrecht. The lower beds 
in the borings seem to have been correctly referred to the Diestian 
series, but one cannot help noticing that southern forms are 
decidedly rarer than they are at Lenham, at any rate in the 
number of individuals. One species, Nucula Cobholdice, is a 
northern mollusc unknown elsewhere in beds older than the upper 
part of the Red Crag, but occurring in profusion in Newer 
Pliocene and Older Pleistocene deposits. It is an extinct form, 
but its invariable association with boreal species in this country 
leaves no doubt that it was a truly northern mollusc. The oldest 
specimens at Utrecht seem to ,be dwarfed and small, as is often 
the case with boreal species towards their southern limit. Not 
one of the southern species belonging to the Diestian beds is 
confined to Holland or especially abundant there. 
